The jack - a square version of the national flag - is used by the Royal
Norwegian Navy.Jan Oskar Engene, 01 May 1996

I have seen a Norwegian cadet ship use the Oslo flag as a jack when at anchor
in San Francisco.

Also, see The Flag Bulletin # 179 p. 33
"In Scandinavian countries, where the jack closely resembles the ensign,
merchant vessels often use the flag of the home port as a jack, a pleasant
contribution to flag usage."James Ferrigan, 11 August 1998